Insoluble dyes have been used as pigments in prior-art electrographic liquid developers. However, processes using such prior-art developers have generally suffered from the poor dispersion stability of such developers and/or poor light stability of the developed image. Prior-art processes which use carbon as the pigment in electrographic liquid developers yield black developed images with good visual density at 550 nm. but have generally suffered from such problems as batch-to-batch nonuniformity and yield poor continuous tone images on print materials such as vesicular elements. Thus, there is a continuing need for insoluble dyes which can be used as pigments in electrographic liquid developers to provide such developers with improved dispersion stability and to provide developed images having improved stability to light. In particular, there is a need to provide an insoluble dye having the desirable neutral density color characteristics of carbon without the previously mentioned problems of carbon when used in liquid developers.
The azo dyes of the present invention can be used to make electrographic liquid developers which have good dispersion stability, which yield good light stability of developed images, and which can provide good continuous tone images and good batch-to-batch uniformity. Electrographic liquid developers made using the azo dyes of the present invention further provide high-quality developed images having low contrast and high resolution.
Various azo dyes containing naphthalene and hydroxynaphthalene groups are described in U.S. Pat. Nos. 1,718,882, 2,244,339, 2,553,261, 2,758,109, 3,384,632, 3,580,901 and 3,781,208, British Pat. No. 1,370,197, and Canadian Pat. No. 926,681. Few of these dyes appear to exhibit color in the blue region of the spectrum. Most of them are more warmly colored, exhibiting orange-red, red-magenta, and brown hues. The azo dyes of the present invention differ structurally from those discussed above and generally exhibit blue or neutral density coloration.